Certain documents such as bank notes are fabricated with security threads having a conducting metal coating thereon which can be detected and are difficult to copy. There are various security devices which are able to detect the presence of these security threads by passing them beneath a magnetic head. The induction of the magnetic head changes when a material characterized by high magnetic permeability, such as a security thread, is placed in the magnetic field of the magnetic head, and in close proximity to the magnetic head. The induction head is part of the electric circuitry so that any changes in its induction causes respective changes in the operating characteristics of the circuitry. Most magnetic heads used for detection of security threads, contain a ferromagnetic core used to enhance the magnetic field in close proximity to the bank note pathway.
Although the ferromagnetic magnetic material enhances the magnetic field and allows concentration of the magnetic field, this enhancement changes as a function of ambient temperature, pressure and extraneous magnetic field. These factors have an impact on the magnetic permeability of the ferromagnetic cores and thus, some changes in the circuitry signal may be caused by changes in these factors, as opposed to the presence of a material which has high magnetic permeability.
The present invention departs from the accepted approach of using a ferromagnetic material to enhance the magnetic field as is common in the prior art. The present structure uses a combination of components and a unique concentrator for producing a secondary magnetic field which can be placed in close proximity to a bank note, for detecting of security threads or other magnetically permeable security structures.